US3406042A - Process for corrosion control - Google Patents

Description

United States Patent 3,406,042 PROCESS FOR CORROSION CONTROL Arthur R. Belyea, Norwalk, Conn., assignor to Consolidated Edison Company of New York, Inc., New York,

N.Y., a corporation of New York No Drawing. Filed Dec. 14, 1965, Ser. No. 513,821 9 Claims. (Cl. 117-134) ABSTRACT OF THE DISCLOSURE Process of inhibiting corrosion in steam generating boilers, and the like, using quaternary ammonium salts.

This invention is concerned with a new and useful process for the inhibition of corrosion of metallic parts which are subject to contact by products of combustion of sulfur containing fuels. More particularly, it is con cerned with the inhibition of acid corrosion caused by deposition of sulfuric acid arising from condensation of combustion products on the surfaces of metallic parts which are reactive with sulfuric acid.

Fuels commonly used in commercial combustion equipment, for example boilers for generating steam, may contain varying proportions of sulfur. Many solid fuels, for example contain from about 0.5 to 3% sulfur. Liquid petroleum fuels may contain from about 2 to about 4 /2 sulfur. When these fuels are burned in the presence of oxygen the principal products of combustion which appear in the flue gases are water, carbon dioxide, sulfur dioxide and sulfur trioxide. Normally the combustion products are not a corrosion problem While the boiler is operating on a high load since their temperature is well above the condensation temperature of even the least volatile component.

Sulfuric acid arises in the flue gases as a result of sulfur trioxide dissolving in water which condenses from the flue gases when their temperature is at or below the dew point, i.e. the acid-water saturation temperature. This may occur during the period when the boiler is starting up but before the temperature of the flue gases exceeds the dew point. It may also occur While the boiler is being banked. During this period the temperature of the flue gases decreases and may reach the dew point, i.e. 350- 400 F.

As a matter of economy, many large boilers are equipped in the flue stack with economizers or with air heaters. The function of these devices is to abstract the heat content from the flue gases so as to save combustion costs. An economizer functions by preheating the water which is to be converted to steam in the boiler. The air heater functions by heating the air which is to be used as the source of oxygen in the combustion of the fuel. These units are generally constructed of iron or other sulfuric acid sensitive metals. The condensation of sulfuric acid on their surface produces ferrous sulfate which because of its poor heat transfer properties compared to iron materially decreases the efficiency of the unit.

In many commercial operations the boiler may be started up or banked several times in the course of an extended period of operation. For example, if the boiler "ice is one which is utilized to service a large urban area containing many manufacturing plants or offices as well as residential sections, the total steam which must be produced in the course of a normal day will reach peak during working hours. The need for steam is materially de- Often a boiler will be shut down altogether for any of a number of reasons. During this period sulfuric acid which deposited on the metallic surfaces while the flue gases were cooling may severely attack the metallic surfaces in the flue stack.

In the past it has been the practice to wash the ferrous sulfate from the metallic surfaces from time to time with aqueous sodium hydroxide or other caustic. Depending upon the particular fuels utilized, the combustion efficiency, the length of the stack and other operating factors it is often necessary to shut down a boiler for a caustic wash as often as once a week. Only rarely is it possible to go for a period of six weeks or more without a caustic wash. This procedure is inefiicient since it takes the boiler out of service, it is costly and, more important, it does not prevent corrosion, it simply removes the products of the corrosion.

It is known to use tertiary amines.particularly coal tar bases obtained by distillation of crude coal tar to inhibit the corrosion of metallic surfaces which come in contact with combustion products. These coal tar bases are principally impure mixtures containing pyridine and its homologs such as collidine and ethyl pyridine as the compounds. The results which can be obtained by the use of tertiary amines are not completely satisfactory since relatively large quantities of coal tar bases must be employed. Moreover, pyridine and its homologs are particularly characterized by their very noxious odor which occasions an air pollution problem.

It has now been discovered in accordance with this invention that corrosion of the type described above can be controlled by condensing or otherwise depositing a small quantity of a selected quaternary ammonium salt or salts on surfaces which are reactive with sulfuric acid. Extremely minute quantities of the selected salt may be used. It has been found in fact that as little as from 1X10 lbs. per sq. ft. to l 10 lbs. per sq. ft. deposited on the area to be protected will effectively inhibit acid corrosion for extended periods of time. This quantity of salt is markedly below the quantity normally employed with tertiary amines. It is such a small quantity relative to the amount of sulfuric acid deposited that it clearly is not a neutralization reaction. Deposition of as little as 40 to 60 parts of salt per million parts of sulfur containing combustion fuel will extend the period between the requirement for caustic washing of air heaters economizers or other devices in the flue stack by as much as ten times or even more.

A number of suitable quaternary salts are available and may be employed in the process of this invention. The

particular salt or salt mixture may be selected from those quaternary ammonium salts which are stable and substantially non-volatile at the dew point of the combustion products. Thus salts which retain their chemical identity and are solid or liquid at temperatures of about 350 F. to about 400 F. may be employed in the practice of this invention. Quaternary ammonium hydroxides and quaternary ammonium halides are as a class generally not preferred for use in this invention, since many of them do not have the desired characteristics with respect to stability and non-volatility. Particularly preferred quaternary ammonium salts include tri-lower alkyl-monohigher alkyl sulfonates especially para-toluene sulfonates in which the total number of carbon atoms in the higher alkyl groups is about sixteen to twenty. Typical quaternary salts which may be used in the invention include cetyl trimethyl ammonium para toluene sulfonate and stearyl trimethyl ammonium para toluene sulfonate. The brand of cetyl trimethyl ammonium para toluene sulfonate available under the trade name CETATS from Fine Organics, Inc., of Lodi, New Jersey, has been found to be especially suitable.

The selected corrosion inhibitor is conveniently introduced in aqueous suspension or solution utilizing hoses, spray, or other equivalent means of distribution. The mixture may be deposited on the surfaces to be protected during a shut down period. Alternatively it may be deposited during the start up period of the boiler when the temperature of the flow gases is increasing or during a banking operation when their temperature is decreasing. In the latter event it is best to introduce the corrosion inhibitor during the period when the flue gases are at or near the dew point. The inhibitor may also be introduced at a point in the flue stack where the flue temperature of the flue gases is above the decomposition temperature of the quaternary salts or above a point where considerable quantities of the inhibitor would be lost by evaporation. However, if this procedure is used the velocity of the flue gases must be such that the inhibitor is rapidly carried to the surface to be protected before the quaternary salt reaches its decomposition temperature or a temperature at which subtsantial quantities are lost due to volatilization.

Any convenient injection means may be employed, for example, a spray nozzle located at or near the area to be protected. The spray is diffused into the flue gases and thereby effectively dispersed over the surface of the economizer, air heater or comparable device. Suitable aqueous mixtures for use in the invention may contain from about 0.003% to about 1.5 by Weight of quaternary salts.

The following non-limiting examples illustrate the process as applied for the protection of economizers or air heaters, although of course the invention is not limited to the protection of such devices.

Example 1 This example was carried out on commercial boilers which normally operate at a steaming rate of about 200,000 lbs. per hour and are equipped with plate-type air heaters. When the boilers were coming off the line in the evening and the steaming rate was below 90,000 lbs. per hour at 0.003% by weight solution containing 85 grams of cetyl trimethyl para-toluene ammonium sulfonate in 750 gallons of water was injected into the flue gas passage ahead of the air heater during a period of to minutes. The area of the air heater surface to be protected is about 27,000 sq. ft. Accordingly, the addition of the 0.003% solution is the equivalent of 0000007 lb. of inhibitor per sq. ft. of heating area. Based on the estimated oil burned during the period of introduction of the inhibitor 0.00035 lb. per gallon of oil or 0.000044 lb. per pound of oil (44 parts per million). Flue gas temperatures at the inlet air heater range from 500 to 600 F. and at the outlet air heater 350 to 450 F. Prior to this treatment it was normally necessary to remove ferrous sulfate by treatment with alkali about every three weeks. Treatment in accordance with this example has extended the time period between alkali washings for as long as 8 weeks or more.

Example 2 The commercial type steam boiler which normally oper' ates at a steaming rate of about 350,000 lbs. per hour and is equipped with steel tube economizers was used in this example. During the banking period when the boiler was down to about 100,000 lbs. of steam per hour a 0.1% solution of cetyl trimethyl ammonium para toluene sulfate containing 4 lbs. of inhibitor dissolved in 480 gallons of water was pumped into the stack at a rate of 60 gallons per hour for 1 hour. Hence 0.5 lb. of the inhibitor was employed. To insure coverage of the entire economizer two points of addition were used. The economizer surface is calculated to be about 16.000 to 17,000 sq. ft. The lower section of the economizer, about 11,000 sq. ft. was treated with 0.00002 lb. of inhibitor per sq. ft. The upper section where corrosion is most severe was treated with 0.00005 lb. per sq. ft. Based on estimated oil burned during the introduction period the total amount of inhibitor is 0.000043 lb. per gallon of oil or 0.000054 lb. per lb. of oil (54 parts per million). Flue gas temperatures in and out of the economizer are approximately the same as in Example 1. This boiler was operated for six weeks without any indication of a requirement for a soda ash treatment.

Similar results are obtained with stearyl trimethyl paratoluene sulfonate and with mixtures of cetyl and stearyl trimethyl para toluene sulfonate.

What is claimed is:

1. A method of inhibiting corrosion of metallic surfaces caused by deposition of condensed aqueous sulfuric acid solutions on said surfaces from the combustion products arising from combustion of sulfur-containing fuels, which method comprises depositing on said surfaces a quaternary ammonium salt which is substantially non-volatile and stable over the temperature range of from about 350 to 400 F. at which the sulfuric acid solution condenses from said combustion products, said salt being deposited in a small quantity sufficient to inhibit said corrosion.

2. A method according to claim 1 in which the quaternary ammonium salt is deposited by injecting an aqueous mixture containing from 0.003 to about 1.5% by weight of the inhibitor into the flue gases during the period while their temperature is increasing, but before the temperature exceeds the dew point.

3. A method according to claim 1 in which the quater nary ammonium salt is deposited by injecting an aqueous mixture containing from about 0.003% to about 1.5% of the salt into the flue gases during a period while the fuel combustion is being decreased, but before the flue gases have cooled to the dew point.

4. The method according to claim 1 in which the quaternary ammonium salt is deposited during a period when fuel combustion is discontinued.

5. A method of inhibiting corrosion of metallic surfaces caused by deposition of condensed aqueous sulfuric acid solutions on said surfaces from the combustion products arising from combustion of sulfur-containing fuels, which method comprises depositing on said surfaces a quaternary ammonium salt which is selected from the group consisting of cetyl and stearyl trimethyl ammonium para-toluene sulfonates and mixtures thereof, said salt being deposited in a small quantity sufiicient to inhibit said corrosion.

6. A method in accordance with claim 5 in which the quantity of said salt which is deposited is from 1 10 to about 1X 10- lbs. per sq. ft. of surface.

7. A method in accordance with claim 5 in which the quaternary ammonium salt is deposited by injecting an aqueous mixture containing from 0.003% to about 1.5 by weight of the inhibitor into the flue gases during the period while their temperature is increasing, but before References Cited the temperature exceeds the dew point.

8. A method in accordance With claim 5 in which the UNITED STATES PATENTS quaternary ammonium salt is deposited by injecting an 3,306,235 2/1967 Lewls 6t XR aqueous mixture containing from about 0.003% to about 5 1.5% of the salt into the flue gases during a period while FOREIGN PATENTS the fuel combustion is being decreased, but before the 734,190 7/1955 Great Britain.

flue gases have cooled to the dew point.

9. A method in accordance with claim 5 in which the JAMES SEIDLECK, Primary Examine!- quaternary ammonium salt is deposited during a period 10 H AYES, Assistant Examiner when fuel combustion is discontinued.